Heat exchanger for exhaust gas cooling; method for operating a heat exchanger; system with a heat exchanger for exhaust gas cooling

ABSTRACT

A heat exchanger, in particular for cooling the exhaust of a motor vehicle internal combustion engine, is disclosed, the heat exchanger comprising a first partial heat exchanger with at least one first flow channel through which a medium to be cooled is to flow and at least one third flow channel through which a first coolant is to flow, at least one second partial heat exchanger with at least one second flow channel through which a medium to be cooled is to flow and at least one fourth flow channel through which a second coolant is to flow, wherein the at least one first flow channel and the at least one second flow channel are fluidly connected, and the at least one first flow channel and the at least one second flow channel have at least one first specific heat transfer surface and at least one second heat transfer surface, wherein second specific heat transfer surface area, divided by first specific heat transfer surface area, yields a quotient (ψ), the at least one first flow channel having a larger quotient (ψ) than second flow channel.

The present invention relates to heat exchangers, more particularly, for cooling the exhaust gas of an internal combustion engine of a motor vehicle, with at least one flow channel through which a medium to be cooled is to flow, and at least one third flow channel through which a first coolant is to flow, at least one second partial heat exchanger with at least one second flow channel through which the medium to be cooled is to flow and with at least one fourth flow channel through which a second coolant is to flow, wherein the at least one first and the at least one second channel are fluidically connected, and the at least one first flow channel and the at least one second flow channel have at least one first specific heat transfer surface and at least one second specific heat transfer surface.

The invention further relates to methods for operating the heat exchanger according to one of Claims 1-22.

The present invention further relates to a system with at least one heat exchanger according to one of Claims 1-22.

Due to increasingly strict emission regulations, a part of the exhaust gas produced in an internal combustion engine is supplied back to the engine after cooling.

A multistage heat exchanger is known from DE 103 28 746 A1. The heat exchanger has turbulence-generating shape elements in the form of ribs, ridges, bumps or embossings.

A system with two-stage exhaust gas cooling is disclosed in DE 10 2005 029 322 A1. The exhaust gas cooler is arranged on the low-pressure side of a turbo charger. In this case in particular, acidic condensate appears, which leads to corrosion of the exhaust gas cooler.

A two-stage exhaust gas cooler, wherein one stage of the exhaust gas cooler is air-cooled and the other stage of the exhaust gas cooler is cooled by means of a liquid coolant, is known from DE 10 2005 042 396 A1.

A two-stage exhaust gas cooler with a high-temperature circuit and a low-temperature circuit is known from DE 10 2007 005 723.9, as yet unpublished. The high-temperature circuit and the low-temperature circuit are separated in this case by a separating wall.

BRIEF SUMMARY OF THE INVENTION

The problem of the present invention is to optimize a heat exchanger of the type mentioned above with regard to overall installation space and costs. In particular, the problem is to prevent the fouling of the heat exchanger by exhaust gas and the associated performance decrease of the heat exchanger in continuous operation.

The problem is solved by the characteristics of Claim 1.

A heat exchanger is proposed, particularly for cooling the exhaust of an internal combustion engine. The heat exchanger has a first partial heat exchanger with at least one first flow channel through which a medium to be cooled, more particularly, exhaust gas, is to flow, and with at least one third flow channel through which a first coolant, more particularly, an aqueous coolant or air, is to flow.

The heat exchanger further comprises at least one second partial heat exchanger with at least one second flow channel through which a medium to be cooled, more particularly, an exhaust gas, is to flow, and with at least one fourth flow channel through which a second coolant is to flow.

The at least one first and the at least one second channel are fluidically connected, and the at least one first flow channel and the at least one second flow channel have at least one first specific heat transfer surface and at least one second specific heat transfer surface.

The second specific heat transfer surface area divided by the first specific heat transfer surface area yields a quotient ψ, the at least one first flow channel having a larger quotient ψ than the second flow channel.

In this manner one can particularly advantageously achieve a heat transfer surface coming into contact with the exhaust gas in the first partial heat exchanger that is large enough that the fouling from the exhaust gas can settle on this surface without the performance substantially decreasing during continuous operation, and at the same time, the heat transferring surface in the second partial heat exchanger is constructed such that the fouling from the exhaust gas is especially advantageously removed from the exhaust gas heat exchanger by condensed water without the occurrence of corrosion in the second partial heat exchanger, which could lead to nonfunctionality of the heat exchanger.

In an advantageous refinement of the invention, the quotient ψ of the at least one first flow channel takes on values of 1.0-2.5 and/or the quotient of the at least one second flow channel takes on values of 0-1.5.

In an advantageous refinement of the invention, the first flow channel and the second flow channel form one constructive unit. In this manner, the heat exchanger particularly advantageously comprises a continuous flow channel for the first and second partial heat exchangers. The heat exchanger is thereby particularly compact and economical, as well as being more easily installable.

In an advantageous refinement of the invention, the first coolant has a higher temperature than the second coolant. In this manner, a high-temperature circuit and a low-temperature circuit are especially advantageously formed.

In an advantageous refinement of the invention, the at least one first flow channel is constructed like a tube and has a first interior tube wall surface that forms the first heat transfer surface.

In an advantageous refinement of the invention, the at least one second flow channel is constructed like a tube and has a second interior tube wall surface that forms the second heat transfer surface.

In an advantageous refinement of the invention, the at least one first flow channel has first turbulence elements. The at least one second flow channel has second turbulence elements. In this manner, the heat transfer performance between the exhaust gas to be cooled and the coolant can be particularly advantageously increased.

In an advantageous refinement of the invention, the first turbulence elements have a first turbulence element height and/or the second turbulence elements have a second turbulence element height.

In an advantageous refinement of the invention, the first turbulence elements are formed as first dimples or first turbulence plates with first rib segments. The second turbulence elements are formed as second dimples or second turbulence plates with second rib segments. In this manner, the turbulence element can be manufactured particularly easily by stamping or pressing and can be matched to the requirements in the first and second partial heat exchanger—in particular, a large surface area for holding the fouling of the exhaust, and a surface shape in the second partial heat exchanger that brings about a condensation of water and rinsing of the fouling.

In an advantageous refinement of the invention, the first turbulence plates and/or the second turbulence plates comprise the second heat transfer surface. In particular, the areas of the second heat transfer surface are particularly advantageously exposed to exhaust gas from both sides. The first heat exchanger surfaces are acted upon by exhaust gas on one side of the wall and by coolant from the opposite side.

In an advantageous refinement of the invention, the first turbulence elements have a first turbulence element height and/or the second turbulence elements have a second turbulence element height.

In an advantageous refinement of the invention, the first turbulence element height is greater than that of the second turbulence element.

In an advantageous refinement of the invention, a first turbulence element density is defined by the number of first turbulence elements relative to a first length of the first flow channel and/or a second turbulence element density is defined by the number of second turbulence elements relative to a second length of the second flow channel.

In an advantageous refinement of the invention, a first turbulence element thickness is greater than a second turbulence element thickness. A particularly good heat transfer through the material concentration is guaranteed in this manner.

In an advantageous refinement of the invention, a first turbulence element thickness is less than a second turbulence element thickness. A particularly good corrosion resistance is guaranteed in this manner.

In an advantageous refinement of the invention, the heat exchanger is a U-flow heat exchanger. In this case, the exhaust gas flows particularly advantageously into the heat exchanger at one side, flows through it, is deflected by 180° and flows back in the opposite direction.

In an advantageous refinement of the invention, the heat exchanger is an I-flow heat exchanger. The exhaust gas flows into the heat exchanger at one side, flows through it and flows back out of the heat exchanger at the opposite end.

In an advantageous refinement of the invention, the heat exchanger has a third partial heat exchanger for reducing thermal stresses. Because of the relatively short heat exchanger, large bending strains due to the high exhaust gas temperature do not arise.

In an advantageous refinement of the invention, the third partial heat exchanger has ⅛ to ¼ of a heat exchanger length of the heat exchanger.

In an advantageous refinement of the invention, the first partial heat exchanger is arranged between the second partial heat exchanger and the third partial heat exchanger.

In an advantageous refinement of the invention, the first partial heat exchanger and/or the second partial heat exchanger and/or the third partial heat exchanger form a constructive unit. In this manner, the first partial heat exchanger and/or the second partial heat exchanger and/or the third partial heat exchanger can be connected particularly advantageously by means of flanges or can be connected into a constructive unit by means of a single housing. In this manner, final installation in a vehicle can be accomplished particularly quickly and simply.

In an advantageous refinement of the invention, the medium to be cooled, and/or the coolant, flow against or with the current in the first partial heat exchanger and/or in the second partial heat exchanger and/or in the third partial heat exchanger.

Additionally, a method for operating the heat exchanger according to one of Claims 1-22 is proposed. The medium to be cooled, in particular, exhaust gas, condenses out at least water while flowing through the second heat exchanger in order to cleanse the second flow channel of fouling from the medium to be cooled. In this manner, fouling is particularly advantageously removed from the second partial heat exchanger, and performance is kept stable over the long term.

In an advantageous refinement of the invention, the medium to be cooled condenses out at least water substantially at a second coolant temperature of less than 40° C.

Additionally, a system with at least one heat exchanger according to one of Claims 1-22 is proposed. Therein at least one second heat exchanger for cooling an internal combustion engine of a motor vehicle and at least one third heat exchanger for cooling the second coolant are provided.

In an advantageous refinement of the invention, at least one fourth heat exchanger for cooling the first coolant is provided.

In an advantageous refinement of the invention, the third heat exchanger is arranged first, followed by the second heat exchanger, as viewed in the direction of the air flow.

In an advantageous refinement of the invention, the fourth heat exchanger is arranged downstream of the second heat exchanger, as viewed in the direction of air flow.

In an advantageous refinement of the invention, the fourth heat exchanger is arranged adjacent to the second heat exchanger as viewed in the direction of air flow and/or essentially at the same height as the second heat exchanger.

In an advantageous refinement of the invention, the second heat exchanger and the fourth heat exchanger are identical.

In an advantageous refinement of the invention, a first control member for regulating the mass flow of the medium to be cooled and/or for bypassing medium to be cooled around at least one partial heat exchanger is arranged on the inflow side of the first heat exchanger.

In an advantageous refinement of the invention, a second control member for regulating the mass flow of the medium to be cooled and/or for bypassing medium to be cooled around at least one partial heat exchanger is arranged on the outflow side of the first partial heat exchanger and the inflow side of the second partial heat exchanger.

In an advantageous refinement of the invention, the heat transfer surface on the coolant side is adapted to the flow conditions prevailing there. The flow there should be turbulent. The turbulent flow is generated particularly advantageously by adapting the flow cross section and/or by means of turbulence generating elements in this area. Coolant-side ribs and/or winglets are particularly advantageous turbulence generating elements.

In an advantageous refinement of the invention, the turbulence generating means are realized particularly in the second stage, in the low temperature cooler stage. In this manner, the mass coolant flow of the low temperature cooler is markedly smaller than that of the high-temperature cooler.

Additional advantageous configurations of the invention follow from the subordinate claims and the drawing. The subject matter of the subordinate relates both to the heat exchanger of the invention, as well as to the system of the invention and the method for operating the heat exchanger of the invention.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

Embodiments of the invention are represented in the drawing and will be described below in detail, wherein there is no limitation of the invention. What is shown are

FIG. 1, a two-stage exhaust gas cooler;

FIG. 2 a, a cutout of the first or second flow channel with a first heat transfer surface;

FIG. 2 b, a cutout of the first or second flow channel with a second heat transfer surface;

FIG. 3 a, a diagram of the factor γ versus the factor ψ for the first partial heat exchanger;

FIG. 3 b, a diagram of the factor γ versus the factor ψ for the second partial heat exchanger;

FIG. 3 c, a diagram of the factor γ versus the temperature of the second coolant for the second partial heat exchanger.

FIG. 4 a, a sectional representation of a two-stage exhaust cooler in a plate construction with continuous plates;

FIG. 4 b, a plan view of another embodiment of a two-stage exhaust gas cooler in a plate construction with continuous plates;

FIG. 5, continuous flow channels with two corrugated turbulence plates;

FIG. 6, a sectional representation of a continuous flow channel with an inserted turbulence plate in the first partial heat exchanger and with dimples in the form of winglets in the second partial heat exchanger;

FIG. 7 a, b, c, d, additional embodiments of turbulence-generating plates;

FIG. 8, a two-stage exhaust gas cooler in U-flow;

FIG. 9, a system with a two-stage exhaust gas cooler;

FIG. 10, a graph with the advantages of two-stage cooling;

FIG. 11, an additional system with a first control member on the inflow side of the first partial heat exchanger and a second control member on the outflow side of the first partial heat exchanger and the inflow side of the second partial heat exchanger;

FIG. 12, a three-stage exhaust gas cooler;

FIG. 13, a first system with a three-stage exhaust gas cooler;

FIG. 14, a second system with a three-stage exhaust gas cooler;

FIG. 15, a third system with a three-stage exhaust gas cooler;

FIG. 16, a fourth system with a three-stage exhaust gas cooler.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a two-stage exhaust gas cooler 1. The exhaust gas cooler has a first partial heat exchanger 11 and a second partial heat exchanger 12.

Partial heat exchanger 11 has a housing of special steel of aluminum or of plastic. First coolant medium flows into partial heat exchanger 11 via a coolant inlet KE1 and, in a first stage, cools the exhaust gas AE flowing in via the inlet diffuser. The coolant exits via outlet KA1. The already cooled exhaust gas flows farther into second partial heat exchanger 12, where it is farther cooled, and subsequently exits in direction AA via outlet diffuser 3. The second coolant, air or water, for example, flows via additional inlet EA2 into partial heat exchanger 12 and out via outlet EA. Second partial heat exchanger 12 has a housing of special steel of aluminum, or of plastic.

FIG. 2 a shows a cutout of first or second flow channel 21, 22 with a first heat transfer surface 23.

FIG. 2 b shows a cutout of first or second flow channel 21, 22 with a second heat transfer surface 24.

FIGS. 3 a, 3 b and 3 c present three diagrams

The factor γ is a quotient that is formed by dividing the thermal power of the cooler without fouling by the thermal power of the fouled cooler, which has fouling deposits.

The factor ψ is a quotient that is formed by dividing the secondary heat transfer surface area 24 by the primary heat transfer surface area 23.

FIG. 3 a shows a diagram of the factor γ plotted versus the factor ψ for first partial heat exchanger 11. In area 33 with ψ<1, too little secondary surface area 24 is available, and the heat transfer power of the cooler is too low. In area 35 with ψ>2.5, there is obstruction and clogging of the exhaust gas cooler. The optimal range 34 (1≦ψ≦2.5) assures high power with low clogging of the exhaust gas cooler.

FIG. 3 b shows a diagram of the factor γ plotted versus the factor ψ for second partial heat exchanger 12. In range 36 (0≦ψ≦1.5) the performance is optimal and the fouling is washed out well. For ψ>1.5, area 37, there is clogging of second flow channels 22.

FIG. 3 c shows a diagram of the factor γ plotted versus the temperature of the second coolant for the second partial heat exchanger Experiments have shown that at temperatures ≦40° C., fouling is particularly advantageously washed out by condensing water.

FIG. 4 a shows a sectional view of a two-stage exhaust gas cooler 1 in a plate construction with continuous plates 41, 42, 43, 44. Identical features are furnished with the same reference numbers as in the preceding figures.

First flow channels 21, second flow channels 22, third flow channels 41 and fourth flow channels 42 are formed by stacked upper plates with sections 43 and 45 and lower plates with sections 44 and 46. In the illustrated embodiment, the plates can be constructed to be continuous, but can also be connected by a form-fit or a material joint. First turbulence elements 47 in the form of turbulence plates or dimples are arranged in first flow channels 21, Second turbulence elements 48 in the form of turbulence plates or dimples are arranged in second flow channels 22.

The plates are formed of a metal such as special steel or aluminum, or of a different metal. The plates are surrounded by a housing 40.

FIG. 4 b shows a plan view of another embodiment of a two-stage exhaust gas cooler 1 in plate construction with continuous plates. Identical features are furnished with the same reference numbers as in preceding figures.

In contrast to FIG. 4 a, coolant inlets and outlets KE1 and 2 as well as KA1 and 2 are on the same side in FIG. 4 b. Area 11 has flat plates that are soldered or to the rib elements of the first turbulence plates. Area 12 shows a corrugated structure. The height of the corrugation corresponds to half the channel height. The gas-side rib has a reduced height. The height of the corrugation structure is correspondingly reduced. The plate can also form a stamped structure, wherein two plates form a tube bundle.

FIG. 5 shows continuous flow channels 50 with two corrugated turbulence plates 47, 48. Identical features are furnished with the same reference numbers as in the preceding figures.

The rib density of second turbulence plates 48 is greater than that of first turbulence plates 47. Therefore there is no clogging in section 11, and water that washes away fouling is condensed out in section 12. A separating wall 49 separates the two coolant circuits from one another.

FIG. 6 shows a sectional representation of continuous flow channel 60 with an inserted turbulence plate 61 in first partial heat exchanger 11, and with dimples 62 in the form of winglets in second partial heat exchanger 12. Identical features are furnished with the same reference numbers as in the preceding figures.

FIGS. 7 a, b, c, d show other embodiments of turbulence-generating plates. Identical features are furnished with the same reference numbers as in the preceding figures.

FIG. 7 a shows a flat plate 71 with a turbulence plate 70. FIG. 7 b shows two soldered corrugated plates 72, 73. The corrugation structure can also be rounded. FIG. 7 c shows corrugated plates with ribs soldered between them. The corrugation structure can also be rounded. FIG. 7 d shows tube bundles from two stamped plates 74.

FIG. 8 shows a two-stage exhaust air heat exchanger 80 in a U-flow design. Identical features are furnished with the same reference numbers as in the preceding figures.

The exhaust gas cooler has a housing 81 and a deflection element 82.

FIG. 9 shows a system 90 with a two-stage exhaust gas cooler. Identical features are furnished with the same reference numbers as in the preceding figures.

System 90 has a turbocharger 103. Via charge air inlet 96, charge air from the environment is compressed in turbocharger 103, cooled in first charge air cooler 100, further condensed in second turbocharger 104 and again cooled in the second charge air cooler, a high-pressure cooler, and subsequently supplied to engine 95.

The exhaust gas arising in engine 95 flows through line 97. A line 99 conducts a part of the exhaust gas via turbochargers 104, 103 to the exhaust pipe; another part of the exhaust gas is fed back in line 98 and, before that, cooled in heat exchanger 1 in first stage 11 and then in second stage 12, and mixed in with the cooled charge air. Second charge air cooler 94 and second partial heat exchanger 12 are supplied by low-temperature circuit 102 with coolant, which is cooled in low-temperature cooler 93 by air drawn in by fan 91. Between fan 91 and low-temperature cooler 93, coolant cooler 92 is arranged. The latter supplies coolant to engine 95 as well as first partial heat exchanger 11. Air flows in the LR direction through second and third heat exchangers 92, 93.

FIG. 10 shows a graph with the advantages of two-stage exhaust gas cooling. The low-temperature cooler LT-EGR (second partial heat exchanger 12) achieves clearly lower temperatures and scarcely any fouling.

FIG. 11 shows an additional system 110, with a first control member 111 on the inflow side of first partial heat exchanger 1 and second control member 112 on the outflow side of first partial heat exchanger 11 and the inflow side of second partial heat exchanger 12. Identical features are furnished with the same reference numbers as in the preceding figures.

FIG. 12 shows a three-stage exhaust gas cooler with an additional third partial heat exchanger 123. The latter reduces the alternating thermal stresses of sections 11 and 12, and has ¼ to ⅛ the overall length of the heat exchanger. Part 123 can be operated with co-current or countercurrent gas flow and cools the gas down to 300-400° C. There is a high gas flow rate and a low pressure drop on the gas side because of the low number of ribs and the few turbulence-generating structures, Smooth ribs or only a few winglets are formed, so that there is a low rib density. A third circuit 133, with a temperature level above that of the engine coolant such as propylene glycol at 160° C. to 200° C. This yields a performance increase with an appropriate arrangement of the recooler.

The most heat is removed from the exhaust gas in section 11, but there must be no clogging due to fouling.

The desired temperature is ultimately reached in section 12. The water contained in the exhaust gas condenses and thus facilitates the cleaning of section 12.

FIG. 13 shows a first system 130 with a three-stage exhaust gas cooler. Identical features are furnished with the same reference numbers as in the preceding figures. In contrast to FIG. 9, a fourth heat exchanger 134 is provided.

FIG. 14 shows a second system 140 with a three-stage exhaust gas cooler. Identical features are furnished with the same reference numbers as in the preceding figures. Second heat exchanger 142 and fourth heat exchanger 144 are arranged at essentially the same height with respect to the LR direction.

FIG. 15 shows a third system 150 with a three-stage exhaust gas cooler. Identical features are furnished with the same reference numbers as in the preceding figures. In this case, a separate second fan 152 is provided for the fourth heat exchanger.

FIG. 16 shows a fourth system 160 with a three-stage exhaust gas cooler. Identical features are furnished with the same reference numbers as in the preceding figures. In this case, the second heat exchanger and the fourth heat exchanger are realized in a single heat exchanger 162.

The heat exchangers of FIGS. 1-16 can be charge air coolers and/or oil coolers and/or coolant radiators in addition to exhaust gas coolers.

The characteristics of the various embodiments can be combined with one another in any desired manner. The invention can also be used for fields other than those shown. 

1. A heat exchanger for cooling the exhaust of a motor vehicle internal combustion engine, comprising a first partial heat exchanger with at least one first flow channel through which a medium to be cooled is to flow and at least one third flow channel through which a first coolant is to flow, at least one second partial heat exchanger with at least one second flow channel through which a medium to be cooled is to flow and at least one fourth flow channel through which a second coolant is to flow, wherein the at least one first flow channel and the at least one second flow channel are fluidly connected, and the at least one first flow channel and the at least one second flow channel have at least one first specific heat transfer surface having a first specific heat transfer surface area and at least one second heat transfer surface having a second specific heat transfer surface area; wherein the second specific heat transfer surface area, divided by the first specific heat transfer surface area, yields a quotient (ψ), the at least one first flow channel having a larger quotient (ψ) than second flow channel.
 2. The heat exchanger according to claim 1, wherein the quotient (ψ) of the at least one first flow channel has a value of 1.0-2.5 and/or the quotient (ψ) of the at least one second flow channel has a value of 0-1.5.
 3. The heat exchanger according to claim 1, wherein the first flow channel and second flow channel form a constructive unit.
 4. The heat exchanger according to claim 1, wherein the first coolant has a higher temperature than the second coolant.
 5. The heat exchanger according to claim 1, wherein the at least one first flow channel is tubular and has a first tube interior surface that forms the first heat transfer surface.
 6. The heat exchanger according to claim 1, wherein the at least one second flow channel is tubular and has a second tube interior surface that forms the first heat transfer surface.
 7. The heat exchanger according to claim 1, wherein the at least one first flow channel has first turbulence elements and/or the at least one second flow channel has second turbulence elements.
 8. The heat exchanger according to claim 7, wherein the first turbulence elements have a first turbulence element height and/or the second turbulence elements have a second turbulence element height.
 9. The heat exchanger according to claim 7, wherein the first turbulence elements are first dimples or first turbulence plates with first rib segments and/or the second turbulence elements are second dimples or second turbulence plates with second rib segments.
 10. The heat exchanger according to claim 7, wherein the first turbulence plates and/or the second turbulence plates have the second heat transfer surface.
 11. The heat exchanger according to claim 9, wherein the first turbulence elements have a first turbulence element height and/or the second turbulence elements have a second turbulence element height.
 12. The heat exchanger according to claim 8 wherein the first turbulence element height is greater than the second turbulence element height.
 13. The heat exchanger according to claim 7, wherein a first turbulence element density is defined by the number of first turbulence elements relative to a first length of first flow channel and/or a second turbulence element density is defined by the number of second turbulence elements relative to a second length of second flow channel.
 14. The heat exchanger according to claim 7, wherein a first turbulence element thickness is greater than a second turbulence element thickness.
 15. The heat exchanger according to claim 7, wherein a first turbulence element thickness is less than a second turbulence element thickness.
 16. The heat exchanger according to claim 1, wherein the heat exchanger is a U-flow heat exchanger.
 17. The heat exchanger according to claim 1, where in the heat exchanger is an I-flow heat exchanger.
 18. The heat exchanger according to claim 1, wherein the heat exchanger has a third partial heat exchanger for reducing thermal stresses.
 19. The heat exchanger according to claim 18, wherein the third partial heat exchanger has ⅛ to ¼ of a heat exchanger length of the heat exchanger.
 20. The heat exchanger according to claim 18, wherein the first partial heat exchanger is arranged between the second partial heat exchanger and the third partial heat exchanger.
 21. The heat exchanger according to claim 18, wherein the first partial heat exchanger and/or second partial heat exchanger and/or third partial heat exchanger form a constructive unit.
 22. The heat exchanger according to claim 18, wherein the medium to be cooled, and/or the coolant, flow with or against the current in first partial heat exchanger and/or in second partial heat exchanger and/or in third partial heat exchanger.
 23. A method for operating a heat exchanger comprising a first partial heat exchanger with at least one first flow channel through which a medium to be cooled is to flow and at least one third flow channel through which a first coolant is to flow, at least one second partial heat exchanger with at least one second flow channel through which a medium to be cooled is to flow and at least one fourth flow channel through which a second coolant is to flow, wherein the at least one first flow channel and the at least one second flow channel are fluidly connected, and the at least one first flow channel and the at least one second flow channel have at least one first specific heat transfer surface having a first specific heat transfer surface area and at least one second heat transfer surface having a second specific heat transfer surface area; wherein the second specific heat transfer surface area, divided by the first specific heat transfer surface area, yields a quotient (ψ), the at least one first flow channel having a larger quotient (ψ) than second flow channel, the method comprising passing exhaust gas to be cooled through the heat exchanger, condensing out at least water while flowing through second heat exchanger, and cleaning the second flow channel from fouling from the exhaust gas.
 24. The method according to claim 23, including condensing out at least water from the exhaust gas substantially at a second coolant temperature of less than 40° C.
 25. A system comprising at least one heat exchanger comprising a first partial heat exchanger with at least one first flow channel through which a medium to be cooled is to flow and at least one third flow channel through which a first coolant is to flow, at least one second partial heat exchanger with at least one second flow channel through which a medium to be cooled is to flow and at least one fourth flow channel through which a second coolant is to flow, wherein the at least one first flow channel and the at least one second flow channel are fluidly connected, and the at least one first flow channel and the at least one second flow channel have at least one first specific heat transfer surface hang a first specific heat transfer surface area and at least one second heat transfer surface having a second specific heat transfer surface area; wherein the second specific heat transfer surface area, divided by the first specific heat transfer surface area, yields a quotient (ψ), the at least one first flow channel having a larger quotient (ψ) than second flow channel; at least one second heat exchanger for cooling an internal combustion engine of a motor vehicle and at least one third heat exchanger for cooling the second coolant.
 26. The system according to claim 25, further comprising at least one fourth heat exchanger for cooling the first coolant.
 27. The system according to claim 25, wherein the third heat exchanger is arranged first, as viewed in the direction of the air flow, followed by the second heat exchanger.
 28. The system according to claim 26, wherein the fourth heat exchanger is arranged downstream of second heat exchanger, as viewed in the direction of air flow.
 29. The system according to claim 26, wherein the fourth heat exchanger is arranged adjacent to second heat exchanger as viewed in the direction of air flow (LR) and/or essentially at the same height as second heat exchanger.
 30. The system according to claim 26, wherein the second heat exchanger and the fourth heat exchanger are identical.
 31. The system according to claim 25, further comprising a first control member for regulating the mass flow of the medium to be cooled and/or for bypassing the medium to be cooled around at least one partial heat exchanger, arranged on the inflow side of the first heat exchanger.
 32. The system according to claim 31, further comprising a second control member for regulating the mass flow of the medium to be cooled and/or for bypassing medium to be cooled around at least one partial heat exchanger arranged on the outflow side of first partial heat exchanger and on the inflow side of second partial heat exchanger. 